Google Assistant is slowly being rolled out for older Android tablets running on 7.0 Nougat or 6.0 Marshmallow, and some smartphones using Android 5.0 Lollipop in selected countries.
It's now available for devices in the US and will soon be open to Android 5.0. Lollipop phones for English users in the US, UK, India, Australia, Canada and Singapore. Spanish speakers in Spain, Mexico and the US will also be supported as will native-language speakers in Japan, Italy, Germany, Brazil and South Korea.
Previously only phones running Marshmallow and Nougat had access to Google Assistant. Note that the tablet version won't respond to OK Google or run Google Express though, so capability is limited.